1. Field of the Invention
This invention relates generally to high-speed printers and more particularly to tractor systems for moving the print paper through the printer past the printing mechanism.
2. Description of Prior Art
High-speed printers employed as terminal printers for computer readout and word processing systems generally utilize a continuous paper feed system. A tractor system, driven by a stepper motor, engages perforations provided along each edge of the print paper and steps the print paper past the printing mechanism. In addition, the tractors serve to center the paper in the printer and to guide the paper as it is stepped through the printer. In printers utilizing more than one tractor for each edge of the paper, it is necessary to keep the upper and lower tractors on each edge of the paper precisely aligned to prevent skewing and consequential jamming or tearing of the print paper.
The Hewlett-Packard Model 2619A printer tractor system utilizes two pairs of tractors, each pair consisting of an upper and a lower tractor, one pair for each edge of the print paper. Each tractor is mounted on an individual lead screw and the two lead screws corresponding to each pair of tractors are linked to a reversible motor by means of a non-slipping drive such as a cog belt. Limit switches are provided to prevent overdriving the tractor pairs and consequential damage to the drive motors. While this system is highly satisfactory, it does require either two motors or one motor with a rather complex shifting mechanism which adds to the cost thereof. In addition, to facilitate removal of the tractors, the entire lead screw assembly has to be disassembled.
Other printers utilize a manually operated cable system wherein each tractor is mounted on a cable and each pair of upper and lower tractors is linked together by a belt and pulley system. The primary disadvantage of this system is that the relative alignment between the upper and lower tractors is difficult to maintain because of cable slippage on the pulleys. Dirt or other foreign matter in the system create enough drag to result in cable slippage. Any stretch in the cables has to be compensated for by manual (turnbuckle) adjustment. As in other prior solutions, it is difficult to remove the tractors from the drive system.
Cable systems to provide translational and/or rotational motion to a moving element are well known in the prior art. U.S. Pat. No. 3,926,061 issued to Gary L. Paulson on Dec. 16, 1975, teaches a drive system in which a pair of fixedly mounted motors impart simultaneous and independent rotational and translational motion to a disc such as a print disc containing a character font.
It is also well known to utilize cable and pulley configurations in other applications. One such application consists of a cable in a figure-eight configuration mounted on a drafting table by pulleys at each corner of the drafting table. The opposite ends of a straightedge are coupled to opposite sides of the figure-eight in such a manner that the straightedge is maintained parallel with respect to a reference usually one edge of the drafting table, when it is repositioned on the drafting table.